1. Field of the Invention
The present invention is directed to a conductive electrolyte for high voltage electrolytic capacitors and to an electrolytic capacitor impregnated with the electrolyte of the present invention for use in an implantable cardioverter defibrillator (ICD).
2. Related Art
Compact, high voltage capacitors are utilized as energy storage reservoirs in many applications, including implantable medical devices. These capacitors are required to have a high energy density since it is desirable to minimize the overall size of the implanted device. This is particularly true of an Implantable Cardioverter Defibrillator (ICD), also referred to as an implantable defibrillator, since the high voltage capacitors used to deliver the defibrillation pulse can occupy as much as one third of the ICD volume.
Implantable Cardioverter Defibrillators, such as those disclosed in U.S. Pat. No. 5,131,388, incorporated herein by reference, typically use two electrolytic capacitors in series to achieve the desired high voltage for shock delivery. Electrolytic capacitors are used in ICDs because they have the most nearly ideal properties in terms of size, reliability and ability to withstand relatively high voltage. Conventionally, such electrolytic capacitors include an etched aluminum foil anode, an aluminum foil or film cathode, and an interposed kraft paper or fabric gauze separator impregnated with a solvent-based liquid electrolyte. While aluminum is the preferred metal for the anode plates, other metals such as tantalum, magnesium, titanium, niobium, zirconium and zinc may be used. A typical solvent-based liquid electrolyte may be a mixture of a weak acid and a salt of a weak acid, preferably a salt of the weak acid employed, in a polyhydroxy alcohol solvent. The electrolytic or ion-producing component of the electrolyte is the salt that is dissolved in the solvent. The entire laminate is rolled up into the form of a substantially cylindrical body, or wound roll, that is held together with adhesive tape and is encased, with the aid of suitable insulation, in an aluminum tube or canister. Connections to the anode and the cathode are made via tabs. Alternative flat constructions for aluminum electrolytic capacitors are also known, comprising a planar, layered, stack structure of electrode materials with separators interposed therebetween, such as those disclosed in the above-mentioned U.S. Pat. No. 5,131,388.
In ICDs, as in other applications where space is a critical design element, it is desirable to use capacitors with the greatest possible capacitance per unit volume. Since the capacitance of an electrolytic capacitor is provided by the anodes, a clear strategy for increasing the energy density in the capacitor is to minimize the volume taken up by paper and cathode and maximize the number of anodes. A multiple anode stack configuration requires fewer cathodes and paper spacers than a single anode configuration and thus reduces the size of the device. A multiple anode stack consists of a number of units consisting of a cathode, a paper spacer, two or more anodes, a paper spacer and a cathode, with neighboring units sharing the cathode between them.
Typically, an implantable cardioverter defibrillator may utilize two 350 to 400 volt electrolytic capacitors in series to achieve a voltage of 700 to 800 volts. However, with the prospect for treatment of ventricular tachycardia with higher voltage pulses (up to 1000 volts), the need for a capacitor with a working voltage of greater than 400 volts becomes pronounced. There are numerous commercially available compositions of electrolyte for use in electrolytic capacitors that can conform to reasonable specifications, as long as the operating voltage of the capacitor remains at 400 volts or lower. However, once this limit is exceeded, the choices become more limited. Electrolytes that have working voltages of greater than 400 volts and high conductivities are in short supply.
There are relatively few electrolytes for this voltage range, and the suitable electrolytes known in the art have several drawbacks, especially when used in a flat, stacked capacitor having a multiple anode configuration, First, glycol-based electrolytes suffer from relatively poor conductivity and ionic mobility. These electrolytes will produce a capacitor with significant energy loss due to a higher than acceptable equivalent series resistance (ESR). Second, xcex3-butyrolactone based electrolytes, which overcome the problems of ionic mobility, can not be used in conjunction with typical paper spacer pads. These require thicker, more expensive pads made out of manila fibers, and as a result of greater thickness, sharply reduce the energy density in flat stacked capacitor designs.
Many high voltage electrolytes employ the use of very long chain dicarboxylic acids and large bases to achieve the necessary breakdown voltages, however, the resultant electrolytes have very low conductivities (xe2x89xa61 mS/cm). For example, U.S. Pat. No. 4,860,169 to Dapo discloses an electrolytic capacitor for use in operation at voltages above 500 volts, produced by employing an electrolyte containing a straight chain saturated aliphatic dicarboxylic acid in which the carboxylic moieties are separated by at least 14 carbon atoms in a mixture of at least one polar organic solvent and water. The disclosed composition has a resistivity at 30xc2x0 C. of 1280 xcexa9xc2x7cm (781 xcexcS/cm), a pH of 9.68, Scintillation voltage of 500V and viscosity of about 14 cP.
Of these existing systems, most achieve reasonable breakdown voltages and high conductivity at the expense of viscosity. What is needed in the art is an electrolyte, for use in a flat capacitor with a multiple anode stack configuration, with high conductivity and breakdown voltage, but which also has a low viscosity, allowing it to reach all portions of the anode surface area with a conductive pathway. Typical electrolytes consisting solely of ethylene glycol and one or more dicarboxylic acids, such as sebacic, azelaic, or suberic acid exhibit viscosities in excess of 15 centipoises at 37xc2x0 C., depending on the amount of water present in the electrolyte. Modest gains in viscosity can be achieved by increasing water content, but often at expense of electrolyte breakdown voltage and oxide stability over time within the finished capacitor.
The present invention is directed to a conductive electrolyte for use in high voltage electrolytic capacitors in which the viscosity is modified using one or more cosolvents, and to an electrolytic capacitor impregnated with the electrolyte of the present invention for use in an implantable cardioverter defibrillator (ICD). Either in lieu of or in addition to water inclusion, one or more cosolvents are added to an electrolyte mixture to reduce the total final viscosity of the solution.
Accordingly, the electrolyte according to the present invention comprises a two solvent mixture of ethylene glycol and a polar organic cosolvent. Preferred cosolvents consist of any of several alcohols including C1-C4 alkanols, such as ethyl alcohol, propyl alcohol, isopropyl alcohol, and butanol, as well as the alkoxy alcohols, such as the C1-C4 alkoxy alkanols, including the alkoxy substituted ethanols (known as xe2x80x9cCellosolvesxe2x80x9d) 2-methoxyethanol, 2-ethoxyethanol, and 2-butoxyethanol. Further, dissolved in this mixture is a combination of: a high dielectric cosolvent (such as acetonitrile, propylene carbonate or dimethyl sulfoxide (DMSO)), a long chain monocarboxylic acid (C12 to C18) (such as lauric acid or stearic acid), and an aliphatic dicarboxylic acid of carbon chain length from eight to thirteen (C8 to C13) (Such as suberic, azelaic, sebacic, undecanedioic, dodecanedioic, or brassylic acid). The solution is then neutralized with an amine, such as ammonia, dimethylamine, trimethylamine, diethylamine, triethylamine, ethanolamine, diethanolamine, triethanolamine, and diisopropylethylamine. A cathode depolarizer, or degassing agent, from the group of nitro-substituted aromatic compounds (nitroaromatics), including nitrobenzene, nitrotoluene, nitrophenol, nitroacetophenone, nitrobenzyl alcohol, and nitroanisole, can be optionally added to reduce the amount of gas produced during capacitor life. Lastly, hypophosphorous acid can be optionally added to enhance the life characteristics of the electrolyte, resulting in lower leakage currents and better voltage droop characteristics. The water content can be adjusted with deionized water to achieve a Karl Fischer titration (water content) measurement of about 1.0% to about 8.0% to achieve proper age characteristics.
A representative composition according to the present invention that displays the desired properties is:
69.6% by weight ethylene glycol;
10.7% by weight 2-methoxyethanol;
5.8% by weight acetonitrile;
3.8% by weight azelaic acid;
1.9% by weight lauric acid;
3.9% by weight ammonium hydroxide (30% in water);
0.1% by weight hypophosphorous acid (50% in water)
1.0% by weight o-nitroanisole; and
3.2% by weight deionized water.
The electrolyte according to the present invention, when impregnated in an electrolytic capacitor, has high conductivity and breakdown voltage, while maintaining a low viscosity, allowing it to reach all portions of the anode surface area with a conductive pathway. Due to the low viscosity, this electrolyte system can be used in multianode per layer stacked configuration aluminum electrolytic capacitors with greater total capacitance realization. Additionally, these electrolytes will result in capacitors with greater stability due to the fact that low amounts of water can be used, while achieving low viscosity.